Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, which heats and subsequently ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece because the impedance of the workpiece to ground is lower than the impedance of the torch tip to ground. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
During the pilot arc and transferred arc modes, or just prior to and in the cutting mode, the gas flow is commonly referred to as “hot flow”, whereas prior to arc ignition the gas flow is commonly referred to as “cold flow” (when no arc is present). During both hot flow and cold flow, the plasma arc torch presents a restriction to the gas flow, which is greater during hot flow than during cold flow. Thus, during hot flow, the flow rate of the gas through the plasma arc torch is lower than the flow rate of the gas during cold flow and the pressure is higher. Additionally, the flow rate decreases and the pressure increases with increasing current level since the increased current introduces a greater restriction within the plasma arc torch. In operation, when the gas flow transitions from cold flow (no arc) to hot flow (arc present), the gas flow rate drops rapidly due to the greater restriction and then slowly returns to the desired level, which is typically set prior to arc ignition. Therefore, a cold flow must be set that results in the desired hot flow once the arc is established. As a result, the first few seconds of the cut, while the flow is recovering, is typically less than optimum quality.
The flow rate of the gas also affects arc voltage such that when the gas flow rate fluctuates, the arc voltage also fluctuates. This can be problematic in plasma arc cutting systems that use arc voltage as a feedback signal to control torch standoff, or height above the workpiece being cut. The proper torch height will not be maintained until the flow has stabilized, and as a result, a less than optimum quality cut occurs when the gas flow fluctuates and the torch height is not properly set. Additionally, the low gas flow rate when transitioning from cold flow to hot flow often causes a double arcing condition, wherein the arc transfers back to the plasma arc torch from the workpiece, typically at the tip, which damages the tip orifice and also produces less than optimum quality cuts.